Jumeirah beach and Al Mamza get my vote. Jumeirah beach goes for miles. There is a beach park and it costs a few dirhams to get in. One day is ladies only. I would get myself a guide book if I were you.
The last couple of books, I bought at Duty Free in Dubai. They were Time Out books. One was about leisure activities in the UAE. Top ten beaches were featured. There are loads of different books around like this. You will find them in supermarkets and book shops everywhere.